After closing three times due to lockdowns and weekend curfews, ABAA is now back to meeting and conducting activities on Sunday evenings at Planetarium. Last Sunday at ABAA there was a talk on the James Webb Space Telescope given by Ravindra Aradhya, President of ABAA. This was the part one of the series and concentrated on the optics of James Webb Telescope. The idea behind the talk was on how Amateur Astronomers can relate their making of telescopes with JWST.
As all telescope makers know, the first and most important part is selecting and buying glass blanks. The selection of glass blanks is done based on the need, thermal expansion is one most important of all the criteria. From Speculum metal, used by Newton and William Herschel, Plate glass, a familiar glass to all amateurs, Pyrex used by many amateurs and observatories to Zerodur used by observatories. The talk covered in detail on the advantages and disadvantages of each type of mirror.
Beryllium used in the making of the James Webb Space telescope was chosen again with the same criteria. The ability of Beryllium to keep the optical shape intact in different temperatures, this is something all Amateur Telescope Makers (ATM) can relate to and has first had experience in that.
The next concern is the weight, Beryllium was chosen keeping this also as priority. At ABAA members are very familiar with J. Texerea’s mirror diameter to thickness ratio of 1:6. This is followed for smaller mirrors but the ratio of 1:10 is considered from larger mirrors. This is done to reduce the weight of the primary mirror. For observatory class mirrors, weight is reduced with hollow mirrors with honeycomb support. We can see this kind of mirror used in Hale telescope at Palomar Observatory.
After covering the topic of choice of glass and weight reducing methods in mirror making the talk covered in detail on the shape of the James Webb Space Telescope, the Hexagon, The geometry and advantages of multi mirrors with Hexagon mirrors was discussed in detail. When it comes to aligning the mirrors, amateurs can understand the importance and difficulties in collimating a telescope. Many were fascinated to know that the hexagon mirrors are already being used in keck telescopes
Next was on the topic of type of telescopes and amateur astronomers know the two kinds of optics most used, the Newtonians and the Cassegrains. The optics and light path was discussed in detail and members could relate the light path in James Webb telescope. Little more time was spent on understanding theThree-mirror anastigmat optics of James Webb.
It became easier for members to relate their telescope making experience of grinding, polishing and testing. The method of polishing, difficulties and the precision is something that amateur telescope makers are too familiar with. Members now could appreciate and admire the engineering that went into making of JWST.
The surface error that was allowed and achieved for JWST mirrors was 20 nanometers. This number does not make much sense, it's just a number. Until we convert the 1.3 meter segment into nanometers, this will be 1,300,000,000 nanometers, now there were a lot of raised eyebrows in the room. The deformity on the surface should not exceed 20 nanometers at temperature of 40 kelvin (-233 C). To visualize this, our DNA is just 2 nanometers in diameter. Members and audience were excited to realize how small an error astronomers and amateurs work on.
Finally the topic of coating was discussed. Amateur telescope makers, after polishing and testing their mirrors, send the mirror for aluminum coating. Different kinds of coating
and their applications were discussed. The sliver, aluminum and gold coating is used for different wavelengths. Since JWST is going to study mainly in InfraRed, gold is the best choice. The talk later covered briefly the process of coating mirrors.
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